The field of art to which this invention pertains is processes for the polymerization of ethylenically unsaturated compounds, particularly polymerization processes involving the use of mercaptans.
Mercaptans are well known activators and chain transfer agents for the polymerization of ethylenically unsaturated compounds. As described in "Vinyl and Related Polymers", by C. E. Schildknecht, John Wiley & Sons, Inc., New York (1952) at pages 98-100, the use of a mercaptan regulator in the preparation of GR-S synthetic rubbers resulted in rubbers having greater softness and plasticity than the German Buna S rubber. In the aqueous emulsion polymerization of butadiene and styrene, mercaptans, such as lauryl mercaptan, were used in amounts from about 0.1 to 3.0 weight percent based on the weight of the monomers. During the course of the copolymerization, the lauryl mercaptan was gradually used up by combination with the polymer. GR-S synthetic rubber has been found to contain roughly one sulfur atom per macromolecule.
The radical addition of mercaptans to carbon-carbon unsaturates ["Organic Reactions," Volume 13, John Wiley & Sons, Inc., New York (1963), pages 164-190] is initiated by the usual free radical initiators, such as oxygen, peroxides, azonitriles, and ultraviolet radiation. The reaction is generally considered to proceed via a multistep chain mechanism: ##EQU1## The generation of RS from the mercaptan is accomplished by means of the decomposition of a peroxide, an azonitrile, or other radical-generating species in the reaction mixture, by irradiation of the thiol with X-rays, gammma rays or beta rays, or by direct photolysis with ultraviolet radiation. When the addition of mercaptans to olefins is carried out with oxygen supplied to the reaction mixture, oxygen participates in the reaction and the initial product is a .beta.-hydroperoxy sulfide. The hydroperoxide usually rearranges in the eaction mixture to the isomeric .beta.-hydroxy sulfoxide which is the major product isolated.
U.S. Pat. No. 2,199,799 describes the reaction of thiophenols and acrylic esters in equimolar amounts to produce aryl thioethers of alkyl isobutyrates and alkyl substituted acrylic ester dimers. This reaction is conducted under strong light for a period of about one week.
U.S. Pat. No. 2,396,997 describes the preparation of high molecular weight linear polymers of unsaturated organic compounds using conventional polymerization catalysts and mercaptans as modifiers.
U.S. Pat. No. 2,888,442 describes the preparation of gel free polymers and copolymers of butadiene ranging from liquids to materials resembling art gum in texture using large amounts of mercaptans as modifiers. These polymers are prepared by emulsion polymerization using conventional polymerization catalysts.
In U.S. Pat. No. 3,028,367 low molecular weight copolymers of hydroxyalkyl acrylates are made by heating together in solution in a suitable solvent a mixture of one or more hydroxyalkyl acrylates and one or more alkyl acrylates in the presence of a free-radical-liberating agent as catalyst and an organic mercaptan which serves as a molecular weight regulator.